Systems for separating nitrogen from natural gas



30, 1955' P. E. HAYNES 2,716,332

SYSTEM FOR SEPARATING NITROGEN FROM NATURAL GAS Filed April 20, 1950HYDROGEN SULFIDE, CARBON DDXIDE WATER REMOVE IN V EN TOR. Page: .6 M4rlv-t his flTTORNE).

United States Patent SYSTEMS FOR SEPARATING NITROGEN FROM NATURAL GASPierre E. Haynes, Indianapolis, Ind., assignor to Koppers Company, Inc.,a corporation of Delaware Application April 20, 1950, Serial No. 157,141

8 Claims. (Cl. 62-123) The present invention relates to the removal ofnitrogen from natural gas and more particularly to improvements inprocesses of reducing the nitrogen content of natural gas in the courseof recompression during transmission from its source to the place of itsconsumption.

Extensive facilities have been provided, and additions thereto have beencontemplated, for transporting natural gas over long distances,particularly by way of long cross-country pipe lines. Since the value ofnatural gas for use as fuel and for other purposes depends mainly on itshydrocarbonaceous constituents, including principally methane, andsmaller proportions of other alkanes such as ethane, propane, butane andothers, the removal of an incombustible constituent such as nitrogen,particularly before transmission of nitrogenbearing natural gases forlong distances, has become an important consideration.

Besides hydrocarbons and nitrogen, natural gas coming from existingwells is known to contain hydrogen sulfide, carbon dioxide, helium andwater vapor. It is customary to remove hydrogen sulfide and water fromnatural gas in the vicinity of its source, in view of the respectivecorrosiveness and hydrate-forming tendencies of these constituents. Itis also common practice to remove and recover heavy and more readilycondensable hydrocarbons. It has not been customary in actual practiceto remove nitrogen, carbon dioxide and helium from natural gas beforetransmission through pipe lines to distant places of utilization.

Of the aforementioned latter three gases, nitrogen is found present innatural gas in much larger proporn'ons than either of the others. Insome cases, the proportion of nitrogen is as high as eighty percent,depending on the location of a reserve. It has been recognized that theremoval from natural gas of excess nitrogen (excess is considered bysome authorities to be ten percent or more) should result in importantbenefits, and that its presence has certain objectionable features amongwhich are the following:

A. It causes a reduction in heating value and may limit themarketability of the gas if the heating value is less than acceptedstandards for such fuel.

B. It may limit the separation and recovery of the heavier hydrocarbonsfrom the gas for separate sale as natural gasolines and low pressuregas, because the removal of these heavier hydrocarbons causes a furtherreduction in heating value of the residual gas.

C. It imposes a burden on the gas transportation and distributionfacilities by reducing the capacity of such facilities to transportmaximum amounts of combustible gas.

D. It may reduce the efliciencies of operations such as carbon blackproduction and liquefied gas storage for peak load demand.

The present invention includes improvements in processes wherebynitrogen is removed from nitrogenbearing natural gas with efiiciency andeconomy on a commercial scale. The invention further includes processesand means whereby advantages are gained which result from overcoming anyone or more of the abovementioned objections. Not only does the removalof nitrogen result in an increase in fuel value of natural gas, there isalso an improvement in the flow characteristics of the gas as a resultof an appreciable reduction in the specific gravity and a decrease inviscosity. The net effect of removing nitrogen from natural gas is toincrease substantially the volumetric deliverability, and consequentlythe B. t. u. deliverability, of a transmission system. Furthermore, itcan be demonstrated that the economic benefits from nitrogen removalincrease in nearly direct proportion to the percentage of nitrogenremoved and the distance the gas is transported.

For the removal of nitrogen from nitrogen-bearing natural gas, nitrogenseparated from the gas, in the present process, is effectively employedas a cooling medium for condensing out methane, and other hydrocarbonsif present. During the condensation step which is conducted whilemaintaining the natural gas under pressure, uncondensed nitrogenseparated from liquefied methane is expanded with performance ofexternal work, and the required cooling is performed with the thusexpanded nitrogen. Under conditions hereinafter indicated, no othercooling means and no extraneous refrigerants are found necessary.Furthermore the number of heat exchangers in the system may be reducedto a minimum.

In a more specific application of the invention to removal of nitrogenwhile natural gas is in transit in long pipe lines, advantage is takenof the use of the pressure employed to send the gas to its destination,as well as of the nitrogen content of the gas. Provision is made foradjustment of the proportion of nitrogen and for recycling thereof asneeded while maintaining the pressure of the natural gas duringseparation of nitrogen therefrom. Not only is an appreciable saving ofenergy thereby realized but also a saving in the cost of equipment.

The nature of the invention is further described in detail hereinbelowby way of exemplification, and with reference to the accompanyingdrawing in which is shown diagrammatically a preferred arrangement ofapparatus suitable for carrying out steps in the process claimed.

In the figure shown in the drawing, apparatus for continuous removal ofnitrogen from natural gas is illustrated which comprises a heatexchanger 1, a condenser 2 provided with a liquid receiver 3, a turbine4, and piping for the system described hereinbelow.

Gas purification means, represented by the rectangle 5, are provided forthe pretreatment of natural gas, when necessary, for the removal of suchconstituents as carbon dioxide, water vapor, hydrogen sulphide, and theheavier hydrocarbons. Such constituents, when present, are each removedin a well-known or appropriate manner including physical and/ orchemical methods.

A pipe 6 is provided for conducting nitrogen-bearing natural gas underpressure from a source to the aforesaid pretreatment means 5. Fromthence, a pipe 7 conducts the pretreated gas to the warm end of theheat-exchanger 1. A pipe 8 and valves 9 and 10 are provided forbypassing the pretreatment means 5.

Pipes 7 and 8 are connected to piping 11 in the heatexchanger 1. A pipe12 connects piping 11 to the liquid receiver 3 of the condenser 2. Apipe 13 is provided for conducting uncondensed gas out of the condensertubes 14 of the condenser 2. The pipe 13 is connected to a pipe 15provided with a valve 16, and to a pipe 17 provided with a valve 18. Thepipe 15 is connected to piping 19 in the heat-exchanger 1. Pipe 17 andpiping 19 are connected to a pipe20 which leads to the turbine 4.Expanded gas from the turbine 4 is conducted through a pipe 2.1ainto,the cooling space .22 surrounding the condenser tubes 14. Apipe 23connects the space 22 to apipe 24 provided with a valve 25, and to apipe 26 provided with a valve 27. The pipe 26 connects the pipe 23 tothe heatexchanger ll, :and ithe pipe .24 rconnects :the :pipe 23 ;to aheat-exhangerlfi. Azpipe I29 connects the exchanger .28 tothe. pipe 26.

.A (pipe 5.0 is provided for conducting condensate .from the receiver .8of :the condenser 2 through tthe heatexchanger 28 to a pump :31. 'Ifhedelivery pipe .32 :ofthe pump 51 is connected to'piping 33 intheheat-exchanger 1 which 'in turn is connected to :a .pipe .54 ifOl'remitting punificd methanetrom :the; system.

pipe 4.0 connected to :the heat-exchanger 1 serves to conduct separatednitrogen :to recycling :means, and to .a pipe 41 :provided with a valve4 2 which is opened when nitrogen siszemittedrfizomithe system. Forrecycling :nitrogen .through the system,;a :pipe 431havingra valve 44.connects1the .pipe- 4t) =1t0 a compressor 45, and apipe 46 conneststhecornpressor-ds rto'thezpipen. Apipe =47 leading to the .compressorliSand having a valve 48, .is provided for introducing nitrogen into thesystem :wh en needed for thepurposes set forth hereinbelow.

The above apparatus is PIOVidGd tO serve primarily as an auxiliarysystem for the treatment of natural gas to remove nitrogen. The system-is1brought into operation by :connecting :the .pipe *6 :to a source ofnitrogen-bearing natural :gas iundersufiiciently 'high pressure. Whencoming direct from a well or reserve, the gas :usually requires apreliminary purification in which case it is subjected to treatment inpurification means 5, under the incoming pressure, for the removal ofsuch impurities as hydrogen sulphide, carbon :dioxide, and water, andfor the separation-otheavier hydrocarbons.

In natural gas transmission systemsnow in use, the .gas arrives .at arecompression plant at a pressure of about 500 to 550 pounds per squareinch and is recompressed to apressure ofabout 700-to 750 pounds persquare inch. Prior :to recompressionin such transmission systems, theabove-describedapparatus performs its function in a manner tobeiexplained below.

:If the gas contains any one or more of the above-mentioned constituentswhich are to be removed first, if desired, the valve 9 is kept open andthe valve 1 is shut. If the treatment of the'gas for the removal of suchconstituents is not considered necessary, the valve 9 is shut and thevalve 10 in the pipe 8 is opened. Natural gas, preferably substantiallyfree of the above-mentioned impurities and heavierhydrocarbons, passesat the incoming pressure through the piping 11 in the heat-exchanger 1andsthrough-the pipe '12 to the receiver 3 of the condenser 2 whereincooling is'efiected to condense preferably substantially all of themethane and other low-boiling hydrocarbons while under pressure of theincoming natural gas.

The nitrogengas whichseparates from liquefied methane passes out of thecondenser tubes 14 and is conducted through the pipe '13. Part of thenitrogen passes successively-"through-pipes '15 and '19 to'pipe2tl, andpartthrough pipe 1 7 topipe which leads to-theturbine 4 Where it isexpanded to about atmospheric pressure with performance of externalwork. The resulting --cooled nitrogen passes through pipe 21 into thespace 22 around the condenser tubes 14 to serve as the cooling mediumfor the natural gas. The valves i6 and 18 are adjusted so that the gasreceives =t-he desired superheat in the heat-exchanger 1. It ispreferred that the nitrogen remain in gaseous state in theooolin-g'space in the condenser 2. It is also preferred that thetemperature of the nitrogen be sufiiciently lowerednot-onlytorthepurpose of eliminating practically all the --metha-ne fromthe gas in the condenser, but also to subcool liquid methane in theheatexchanger 28 and to maintain it in liquid 'form as it passes fromthe receiver 3 to thepurnp 31.

The subcooled methane, -or purified natural gas, is pumped by the pump31 through the pipe'32 and piping 33 in heat-exchanger 1 from which itemerges in pipe 34 at about atmospheric temperature and is delivered toa transmission line.

The expanded nitrogen passing from the space 22 through pipe 23 ispreferably divided into two streams one of which passes through thevalve 25 in pipe 24 to the heat-exchanger 28 for the purpose indicatedabove, and the other of which passes through pipe 26. The part of thenitrogen serving to 'subcool the methane passes from the heat-exchanger28 through the pipe 29 to join with the balance of expanded nitrogenfrom pipe .23 .passing through the valve '27. The valves 25 and 27 areadjusted to apportion the nitrogen for obtaining the desired srihcooling.inathe zheataexchanger 28.

The nitrogen from pipe 26 is brought into indirect heatexchangerelationship with the nitrogen passing to the turbine 4 in piping 19;with the incoming natural gas in piping 11; and with the outgoingnatural gas in piping 33. The nitrogen emerges from the exchanger lrintothe pipe 40 at about atmospheric temperature. Thenitrogenmay berecycled, :when required :in the system, through the valve 44 in :therpipe=43 rconnectedto the compressor 45 which raises the ,pressure ofthe recycled nitrogen and feeds it through the pipe 46 into the pipe 6at'substantially the tpressureof the incoming natural gas. if nitrogen.=is needed .in the system eat commencementof operations or duringoperation, :it :can .be admitted through pipe 47.. Nitrogen :that is.removed from the ;gas and that .is .not needed in ithe system can bewithdrawn from the system through )the valve 42 in :pipe 41 and can beused in ammonia synthesis :and 'for other purposes.

.It is seen from the.:above:descriptionthat adjustment of the proportionof nitrogen in :the natural gas brought to the system .-can be readilymade to .a preselected percent when necessary, or when desired, and forobtaining substantially complete removal of nitrogen. A maximumproportion of 35 percent nitrogen in'natural gas is .found to rendertheprocess selt-sufiicient. When .the nitrogen in the natural gas undertreatment is present in .a :proportion .above the selected maximum,nitrogen that is separated from the ,gas in .the system .is notrecycled, but is withdrawn from the system through the pipe 41. When theproportion of nitrogen in natural gas from a given source is less .-thanthe selected maximum .propor.tion,.adjustrnent .of the nitrogen contentof such igasis made no this latter proportion as the gas :enters.thesystern at the .pipe :6. .Such .selected proportion .issubstantially continuoushnrnaintainedifdesired, by regulation of valve48 when necessary, and byregulation of the valves 42 and 44. Samples ofthe incoming natural gas .may be taken .at suitable intervals toascertain the nitrogen content .of the :gas. The volumetric addition ofnitrogen to be made to the gas :can :be determined with meters, as iswell understood.

By way of further illustration, the .following specific example of ltheprocess is set .forth:

.Nitrogembearin-g natural .gas which has been preferably pretreated toremove, substantially completely, undesirable constituents other than.nitrogen .as referred to.above,.is passed into the .piping 111 inheat-exchanger 1 at a temperatureof about 127 C. and -.a:pressure of 500to 550 pounds per square inch. ,If .thezincoming gas contains 15 percent nitrogen, .additional nitrogen is mixed with .the gas to bring thenitrogen content :up to, for instance, 35 percent, .while maintaining.thetlattenpressure. The gas .at such pressure is cooled by.nitrogenentering .the condenser 2 at about .83 K. This nitrogen is the nitrogenvthatvis separated from .natural gas in the condenser .2, and that .isconducted through pipe 13, is provided with controlled superheat, andarrives at the turbine 4 at about .K. and about 500 to 550 poundspressure, and is expanded to lower its temperature to the aforesaid'83'K. or preferably a little vaboveits boiling point, or substantially ormaterially below .the boiling point of .methane at the pressure of thenatural The natural gas from which nitrogen is thus removed, passes outof the receiver 3'at about 180 K. under the initial pressure and inliquid form. This liquid is further cooled to about 165 K. by expandednitrogen from the cooling space 22 of the condenser 2. The processednatural gas leaves the heat-exchanger 1 through pipe 34 at about 27 C.When the system is installed in a recompression plant, the pressure ofthe processed gas needs only to be raised from the pressure in thesystern up to or preferably slightly above outgoing transmission linepressure.

The expanded nitrogen from the condenser 2 and heat exchanger 28 broughttogether in heat-exchanger 1, emerges from the latter exchanger at about27 C. and is recompressed to the pressure of the incoming natural gasand recycled through the system, as it is needed, to bring the nitrogencontent of the gas up to the preselected proportion or, as in thisexample, up to about 35 per cent by volume, or so that substantiallyone-third of the volume of the resulting mixed gases is nitrogen.

The invention hereinabove set forth is embodied in particular form andmanner but may be variously embodied within the scope of the claimshereinafter made.

I claim:

1. In a process of removing nitrogen from nitrogenbearing natural gas,steps comprising while passing said natural gas under pipeline pressurethrough a gas separation system, adding nitrogen to said natural gas atsaid pressure to raise its nitrogen content from a given lowerproportion to a higher preselected proportion to obtain upon subsequentseparation and expansion of said nitrogen a lowering of its temperaturematerially below the boiling point of methane at said pressure, coolingthe resulting mixture in said system at said pressure to liquefysubstantially completely all methane while separating uncondensednitrogen from said liquefied methane,-expanding said uncondensedseparated nitrogen with performance of external work to lower thetemperature of said separated nitrogen materially below the boilingpoint of methane as aforesaid and passing the expanded nitrogen intoindirect heat-exchange relationship with said mixture to effect saidcooling to liquefy said methane as aforesaid, and withdrawingsubstantially nitrogen-free natural gas from said system.

2. In a process of removing nitrogen from nitrogenbearing natural gas,steps comprising while passing said natural gas under pipeline pressurethrough a gas separation system, adding nitrogen to said natural gas atsaid pressure to raise its nitrogen content from a given lowerproportion to a higher preselected proportion to obtain upon subsequentseparation and expansion of said nitrogen a lowering of its temperaturematerially below the boiling point of methane at said pressure, coolingthe resulting mixture in said system at said pressure to liquefysubstantially completely all methane While separating uncondensednitrogen from said liquefied methane, expanding said uncondensedseparated nitrogen with performance of external work to lower thetemperature of said separated nitrogen materially below the boilingpoint of methane as aforesaid and passing the expanded nitrogen intoindirect heat-exchange relationship with said mixture to effect saidcooling to liquefy said methane as aforesaid, compressing to saidpipeline pressure expanded nitrogen coming from the aforesaid coolingstep and adding it to natural gas entering said system to adjust itsnitrogen content in the manner and for the purpose aforesaid, andwithdrawing substantially nitrogen-free natural gas from said system.

3. In apparatus for separating nitrogen from nitrogenbearing naturalgas, a heat-exchanger for precooling natural gas under superatmosphericpressure with expanded nitrogen separated from said gas, a condenser forseparating said gas into liquefied methane and uncondensed nitrogenunder said pressure, means for conducting natural gas from saidheat-exchanger into said condenser, means for expanding said uncondensednitrogen with production of external work, means for conducting saiduncondensed nitrogen under said pressure from said condenser to saidexpanding means, said last-named conducting means including controlmeans. for adjustably providing said uncondensed nitrogen withsuperheat, the said expanding means having gas-passage connection to thecooling space of said condenser for exclusive introduc-' tion ofexpanded gas from said expanding means into said cooling space, meansfor conducting expanded nitrogen from said cooling space of saidcondenser to said heatexchanger for precooling said natural gas, andmeans for withdrawing liquefied methane from said condenser.

4. In apparatus for separating nitrogen from nitrogen.- bearing naturalgas, a heat-exchanger for precooling natural gas under superatmosphericpressure with expanded nitrogen separated from said gas, a condenser forseparating saidgas into liquefied methane and uncondensed nitrogen undersaid pressure, means for conducting natural gas from said heat-exchangerinto said condenser, means for expanding said uncondensed nitrogen withproduction of external work, means for conducting said uncondensednitrogen under said pressure from said a condenser to said expandingmeans, said last-named conducting means including control means foradjustably providing said uncondensed nitrogen with superheat, saidcontrol means comprising a valve in said last-named conducting means forcontrolling flow of nitrogen gas directly to said expanding means and avalve-controlled pipe sec tion in said heat-exchangerfOr'bypassing acontrolled portion of nitrogen through said heat-exchanger to saidexpanding means; means for conducting the expanded nitrogen from saidexpanding means into the cooling space of said condenser, the saidcooling space having gas-entry connection exclusively for said expandednitrogen; means for conducting expanded nitrogen from said cooling spaceof said condenser to said heat-exchanger for precooling said naturalgas, means for withdrawing liquefied methane from said condenser, meansfor bringing a portion of said expanded nitrogen from said condenserinto indirect heatexchange relationship with said liquefied methane fromsaid condenser to subcool said liquefied methane, and means forconducting said liquefied methane from said last-named means throughsaid first-named heat exchanger in indirect heat-exchange relationshipwith said expanded, nitrogen from said condenser.

5. In apparatus for separating nitrogen from nitrogenbearing naturalgas, a heat-exchanger for precooling natural gas under superatmosphericpressure with expanded nitrogen separated from said gas, a condenser forseparating said gas into liquefied methane and uncondensed nitrogenunder said pressure, means for conducting natural gas from saidheat-exchanger into said condenser, means for expanding said uncondensednitrogen with production of external work, means for conducting saiduncondensed nitrogen under said pressure from said condenser to saidexpanding means, said last-named conducting means including controlmeans for adjustably providing said uncondensed nitrogen with superheat,means for conducting the expanded nitrogen from said expanding meansinto the cooling space of said condenser, the said cooling space havinggas-entry connection exclusively for said expanded nitrogen; means forconducting expanded nitrogen from said cooling space of said condenserto said heat-exchanger for precooling said natural gas, means forwithdrawing liquefied methane from said condenser, means for compressingexpanded nitrogen to substantially the pressure of said natural gasentering said first-named heat-exchanger, and means for selectivelyadding the recompressed nitrogen to said natural gas for raising thenitrogen content of said gas to a preselected proportion.

6. In a process of removing nitrogen from nitrogenbearing natural gas atpipe line pressures, steps comprising while introducing into acondensing zone at substantially pipe line pressure, natural gas havingless than about One thild *by volume of nitrogen, adding enough nitrogenat said pressure to increase the nitrogen content to substantiallyone-third; while substantially avoiding a drop -1npressure of theresulting mixture of gases separately removing liquid methane andnitrogen gas from said condensing zone and expanding said separatednitrogen With5performance-of external work to 'lower'the temperature ofseparated nitrogen below the 'boilingpoint of methane at said pressure,and passing'the 'expande'd'nitrogen into indirect heat-exchangerelationship with -rri-ixed natural 'gas and nitrogen in said 'zonetoeffect'liquefaction of methane-in themixtureg'thereby obtainingsubstantialiy nitrogen-free natural gas for transmission at "pipe linepressure.

"7. Apparatus for removing nitrogen from precompressednitrogen-'bearingnaturalgas, said apparatus comprising a gas separationsystem, means for introducing said nitrogen-bearing natural gas intosaid system under incoming pipe line-pressure, said system including acondenser for separatingliquid methanefrom said gas at said incomingpipeline pressure, said condenser having a cooling space exclusively forcooling with expanded nitrogen separated from said gas; a pump forpumping separated methane at outgoing pipe line pressure, means forwithdrawing iseparated nitrogen from said system, and a pump forseparately returning separated nitrogen to said system at said incomingpipe line pressure for raising the nitrogen content of said gas to apreselected proportion.

8. Apparatus for removing "nitrogen from precompressed nitrogembearingnatural gas in transmission for long distances from its source to aremote destination, said-apparatus comprising a gas'separationsystem,means for introducing said nitrogen-bearing natural gas into said systemunder pressure employed 'in 'said transmission, adjustable .means."forintroducing nitrogen at said pressure for1admixture1with said gas'to'adjust the proportion of said nitrogen to obtain upon'subsequentseparation :and expansion of said nitrogen 'a lowering of itstemperature below the boiling point of methane at said pressure, saidsystem including a condenser "for separating liquid methane from s'aidgas at said pressure, said-'condenserhaving a cooling space 'forcoolin'gwith expanded nitrogen separated from said gas, and means forWithdrawing separated methane fromv said system for delivery underpressure employed in said transmission.

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1. IN A PROCESS OF REMOVING NITROGEN FROM NITRGENBEARING NATURAL GAS,STEPS COMPRISING WHILE PASSING SAID NATURAL GAS UNDER PIPELINE PRESSURETHROUGH A GAS SEPARATION SYSTEM, ADDING NITROGEN TO SAID NATURAL GAS ATSAID PRESSURE TO RAISE ITS NITROGEN CONTENT FROM A GIVEN LOWERPROPORTION TO A HIGHER PRESELECTED PROPORTION TO OBTAIN UPON SUBSEQUENTSEPARATION AND EXPANSION OF SAID NITROGEN A LOWERING OF ITS TEMPERATUREMATERIALLY BELOW THE BOILING POINT OF METHANE AT SAID PRESSURE, COOLINGTHE RESULTING MIXTURE IN SAID SYSTEM AT SAID PRESSURE TO LIQUEFYSUBSTANTIALLY COMPLETELY ALL METHANE WHILE SEPARATING UNCONDENSEDNITROGEN FROM SAID LIQUEFIED METHANE, EXPANDING SAID UNCONDENSEDSEPARATED NITROGEN WITH PERFORMANCE OF EXTERNAL WORK TO LOWER THETEMPERATURE OF SAID SEPARATED NITROGEN MATERIALLY BELOW THE BOILINGPOINT OF METHANE AS AFORESAID AND PASSING THE EXPANDED NITROGEN INTOINDIRECT HEAT-EXCHANGE RELATIONSHIP WITH SAID MIXTURE TO EFFECT SAIDCOOLING TO LIQUEFY SAID METHANE AS AFORESAID, AND WITHDRAWINGSUBSTANTIALLY NITROGEN-FREE NATURAL GAS FROM SAID SYSTEM.